The invention relates to digital television receivers for vestigial-sideband (VSB) digital television (DTV) signals and, more particularly, to the portions of such receivers used for recovering baseband symbol coding proceeding from intermediate-frequency signals.
The digitization of intermediate-frequency VSB DTV signal and its subsequent demodulation in the digital regime are described in U.S. Pat. No. 5,479,449. This patent entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVERxe2x80x9d issued Dec. 26, 1995 to C. B. Patel and A. L. R. Limberg. Demodulation in the digital regime is performed in U.S. Pat. No. 5,479,449 by converting the digitized intermediate-frequency VSB DTV signal to complex form to be multiplied in a complex digital multiplier by a complex digital carrier signal supplied from look-up tables stored in read-only memory (ROM). To facilitate converting the digitized I-F signal to complex form using a digital Hilbert transform filter, the final intermediate-frequency band is offset a megahertz (MHz) or so from zero frequency, but its uppermost frequency is kept lower than 10 MHz.
Equalization of the digitized baseband symbol coding that results from demodulation is facilitated by choosing a sampling clock of a rate that is a rational multiple of the symbol rate (i. e., is related to symbol rate by a whole number ratio) and that will satisfy the Nyquist criterion. Supplying the complex digital carrier signal from ROM is facilitated by choosing the carrier in the digitized I-F signal to be a submultiple of the system clock signal rate as described by C. B. Patel and A. L. R. Limberg in U.S. Pat. No. 5,606,579 issued Feb. 25, 1997 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVERxe2x80x9d. Such choice of carrier permits perfect wrap-around of cycles of digital carrier when they are conceived as being mapped to the surface of a cylinder with circumference measured by ROM addresses according to a modular arithmetic.
C. B. Patel and A. L. R. Limberg advocate the digital carrier being located at the upper-frequency end of the final I-F signal band in U.S. Pat. No. 5,731,848 issued Mar. 24, 1998 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING NG FILTERS, AS FOR USE IN AN HDTV RECEIVERxe2x80x9d. That is, the vestigial sideband is above full sideband in frequency in the final I-F signal that is digitized. U.S. Pat. No. 5,731,848 discloses there is reason for this choice of carrier, aside from facilitating the use of Ng filters for converting the real final I-F signal to complex form after its digitization. Fast changes in symbol values are converted to lower-frequency variations in the final I-F signal offered for digitization, which alleviates problems of accurately sampling the final I-F signal as the initial step in digitization. Small changes in sampling phase result in larger changes in the zero-frequency demodulated carrier, so there is tighter automatic frequency and phase control (AFPC) of a local oscillator used in converting the radio-frequency (R-F) VSB DTV signal to the final I-F signal.
The equalization of quadrature amplitude-modulation (QAM) digital signals had been a topic of extensive study before digital television broadcasting standards were being formulated in the early 1990""s. The problems of carrier acquisition and of symbol synchronization were separately considered. If synchrodyning is performed in the analog regime and symbol synchronization is performed in the digital regime, this is a natural dichotomy. If the carrier frequency is not locked to symbol rate, this is a natural dichotomy as well. One general approach to receiver design was to demodulate two mutually orthogonal phases of received QAM using a beat frequency oscillator with highly stabilized frequency and with symbol synchronization being performed after demodulation using adaptive equalization techniques based on errors detected from decision results.
An alternative general approach to receiver design was to demodulate two mutually orthogonal phases of received QAM using an oscillator subject to automatic frequency and phase control based on errors detected from decision results. This latter approach was adapted in the Patel et alii patents described above, the phase of digital carrier being adjusted by a method similar to that described by S. U. H. Qureshi in his paper xe2x80x9cTiming Recovery For Equalized Partial-Response Systemsxe2x80x9d, IEEE Transactions On Communications, December 1976, pp.1326-1330.
When automatic frequency and phase control (AFPC) of the local oscillator is based on errors detected from decision results, passband equalization, rather than baseband equalization, is preferred by those skilled in the art of digital communications receiver design. Baseband equalization is carried out after demodulation, so the equalization filters are included in the feedback loop used for AFPC of the local oscillator based on errors detected from decision results. Changes in the filter coefficients of an adaptive equalization filter affect loop delay and can cause jitter in carrier synchronization unless the loop bandwidth is kept narrow. Narrow loop bandwidth compromises the ability to acquire carrier rapidly when first tuning to a transmission channel and the ability to synchronize to shifts in carrier phasing during multipath reception. Passband equalization is carried out previous to demodulation, which avoids the equalization filters being included in the feedback loop used for AFPC of the local oscillator based on errors detected from decision results. A problem with passband equalization is encountered when adaptive equalization filters are used. The error signal based on errors detected from decision results must be modulated onto the digital carrier to convert it from the lowpass filter regime into the bandpass filter regime in which filter coefficient adjustments are to be made. This problem is merely an inconvenience with QAM, which has a symmetrical passband structure. But the problem is more difficult with VSB, which has an asymmetrical passband structure.
The extraction from ROM of digital carrier of a frequency that is a submultiple of symbol rate or is a submultiple of symbol rate of a multiple of symbol rate ties together the problems of carrier synchronization and symbol synchronization in a way that previously has not been fully appreciated. The digital carrier is sampled in a prescribed way in the ROM, and the AFPC of the local oscillator so the final I-F signal can be synchrodyned to baseband automatically controls the timing of sampling vis-à-vis the carrier of the analog final I-F signal. So, adjustment of symbol synchronization after the digital complex demodulation of baseband symbol coding is not required, there being no need to overcome randomness of sampling phase versus the phasing of the carrier used in the synchrodyning procedure. However, symbol synchronization may be imperfect because of lack of uniform group delay across the receiver passband, which will be the case when multipath reception occurs. Any adjustment of symbol synchronization that is actually required in practice can be taken care of by the equalization filtering. In order to keep equalization filtering from being included in the AFPC loop in the bandpass tracker, baseband equalization is employed in the invention described below, rather than passband equalization being employed.
There is still need for stabilization of sample clock rate to lock its phase to symbol rate or a multiple thereof. This is required for maintaining perfect wrap-around of cycles of digital carrier when conceptually they are mapped to the surface of a cylinder with circumference measured by ROM addresses sequentially addressed at sample clock rate. Sample clock rate stabilization can be done by extracting half-symbol-frequency component from symbol coding and multiplying up in frequency to generate sampling frequency. Such procedures for stabilization of sample clock rate are per se already known in digital communications receiver design. These procedures are improved by extracting half-symbol-frequency component from symbol coding in the digital regime, using a digital bandpass filter. This avoids the problems with rapid changes in phase response across the passband of an analog bandpass filter for extracting half-symbol-frequency component from symbol coding.
The invention is embodied in a bandpass phase tracker for automatically sampling at prescribed carrier phases when digitizing a vestigial-sideband intermediate-frequency signal received from an intermediate-frequency amplifier, which said vestigial-sideband I-F signal is modulated in accordance with a baseband symbol code of a prescribed symbol frequency. A local oscillator is included in this bandpass phase tracker for generating analog oscillations at a frequency and phase subject to control responsive to an automatic frequency and phase control signal. The bandpass phase tracker includes heterodyning circuitry for mixing oscillations from the local oscillator with the VSB I-F signal received from the I-F amplifier to generate an analog low-frequency heterodyne signal with its lowest frequencies offset from zero frequency. The bandpass phase tracker includes analog-to-digital conversion circuitry for sampling the analog low-frequency heterodyne signal in accordance with a first sampling clock signal and converting the resulting samples to a complex digital low-frequency heterodyne signal having real and imaginary components. The bandpass phase tracker includes digital demodulation circuitry for synchrodyning the complex digital low-frequency heterodyne signal with a complex digital carrier signal, thereby to complete demodulation of the VSB I-F signal to supply real and imaginary components of a demodulated signal at baseband. Circuitry responsive to the imaginary component of the demodulated signal is provided for generating an automatic frequency and phase control signal for the local oscillator. The bandpass phase tracker includes an envelope detector for detecting envelope variations in the VSB IF signal received from the I-F amplifier to supply an envelope detector response and a frequency-selective filter for receiving the envelope detector response as its input signal. This frequency-selective filter selectively responds in its output signal to frequencies in its input signal that are proximate to a submultiple of the prescribed symbol frequency. The bandpass phase tracker includes sample clock generating circuitry for responding to the submultiple of said prescribed symbol rate contained in the output signal from the frequency-selective filter to generate the first sampling clock signal and a second sampling clock signal, each at a respective controlled rate that is a rational multiple of that submultiple of the prescribed symbol rate. The bandpass phase tracker also includes a carrier signal generator for generating the digital carrier signal for synchrodyning with the complex digital low-frequency heterodyne signal, with the frequency of the digital carrier being controlled by the second sampling clock signal.
In some embodiments of the invention the digital demodulation circuitry comprises a complex digital multiplier for synchrodyning the complex digital low-frequency heterodyne signal to baseband by multiplying it by a complex digital carrier signal. In these embodiments of the invention the carrier signal generator comprises a sample counter, for counting the sample periods in the second sampling clock signal to supply a modular count, and read-only memory addressed by the modular count, for generating real and imaginary components of the complex digital carrier signal.
In other embodiments of the invention the digital demodulation circuitry is simpler, comprising first and second complementors that respond to a modulo-two count of the sample periods in said second sampling clock signal. The first complementor is for generating the real component of the demodulated signal by selectively complementing the real component of the complex digital low-frequency heterodyne signal. The second complementor is for generating the imaginary component of the demodulated signal by selectively complementing the imaginary component of the complex digital low-frequency heterodyne signal.